| Abstract|| |
Aim: The aim of this study is to evaluate the influence of different combinations of irradiance and exposure time for a given radiant exposure on the degree of conversion (DC) and on the mechanical properties of two resin composites: Filtek Z250 and Filtek P90 LS (3M ESPE).
Materials and Methods: The following curing protocols were used: Standard irradiance: 400 mW/cm2 for 60 s; Medium irradiance: 700 mW/cm2 for 34 s and High irradiance: 950 mW/cm2 for 26 s. The DC was measured using Fourier Transform Infrared spectroscopy. Each specimen was submitted to five indentations to evaluate the Knoop microhardness (KHN). The flexural strength (FS) was obtained from the three-point bending test. Cylindrical specimens were prepared for the Diametral tensile strength (DTS) test. Statistical analysis: Data were evaluated using two-way ANOVA and Tukey's test (α = 0.05).
Results: DC and DTS were not influenced by the different curing protocols. For P90, the medium irradiance showed higher values of KHN than the standard irradiance. For Z250, the high irradiance showed higher values of FS than the standard irradiance.
Conclusion: The influence of the different combinations of irradiance and exposure time depends on the resin composite as well as the specifically evaluated mechanical property.
Keywords: Composite resin, degree of conversion, diametral tensile strength, microhardness
|How to cite this article:|
Gonzalez M R, Poskus L T, Filho HS, Perez C R. Influence of irradiance and exposure time on the degree of conversion and mechanical properties of a conventional and silorane composite. Indian J Dent Res 2013;24:719-22
The organic matrix and different curing protocols of dental composites have been extensively studied to diminish the shrinkage stresses at the adhesive interface. These stresses could disrupt the adhesive interface between the composite and tooth structure, leading to marginal gaps, discoloration, microleakage, caries and post-operative sensitivity,  in addition to cuspal deflection and enamel microfractures. 
|How to cite this URL:|
Gonzalez M R, Poskus L T, Filho HS, Perez C R. Influence of irradiance and exposure time on the degree of conversion and mechanical properties of a conventional and silorane composite. Indian J Dent Res [serial online] 2013 [cited 2019 Oct 21];24:719-22. Available from: http://www.ijdr.in/text.asp?2013/24/6/719/127620
A low-shrinkage composite was recently developed to reduce shrinkage stresses as this material polymerizes through a photo-cationic ring-opening to reduce volumetric shrinkage during curing. 
With respect to the photoactivation, it is known that light sources with high irradiance are able to ensure a higher degree of conversion (DC) and better mechanical properties of composites.  However, several studies have shown that the irradiance and radiant exposure (irradiance X exposure duration) delivered to the composite during photoactivation could influence the properties of composites. ,,,
Indeed, a direct relation between radiant exposure and DC has been shown. This relation was not linear as, from a given radiant exposure, the DC remained practically unchanged.  Thus, a radiant exposure of 20-24 J/cm2 has been considered sufficient for an adequate DC and better mechanical properties ,, as a higher radiant exposure could increase the shrinkage stresses of composites. 
Different combinations of irradiance and exposure duration can be used to obtain the same radiant exposure. However, it has been shown that a higher irradiance can result in a polymer with high crosslink density related to the major number of growth centers formed during irradiation.  However, for conventional composites, some studies did not show any influence of the modes of photoactivation with similar radiant exposures on DC ,, or on the superficial hardness , while others studies did. ,,
Despite the lower shrinkage strain of the silorane-based composite, a higher stress rate has been found for this material.  Therefore, different curing protocols could also be used to minimize the effects of the stress shrinkage at the adhesive interface. However, it is unknown how these procedures can influence the conversion degree and mechanical properties of this low-shrinkage material. The aim of the present study was to evaluate the influence of different combinations of irradiance and exposure time for a given radiant exposure on the conversion degree and mechanical properties of a conventional and a low-shrinkage composite.
| Materials and Methods|| |
Two commercially available composite resins were used in this study: Filtek TM Z250 and Filtek TM P90 LS [Table 1]. All specimens were light activated with a light emitting diode (LED) light-curing units (LCU) (Radii, SDI, Victoria, Australia) that had an output irradiance of 950 mW/cm2. The distance between the specimen and the tip of the LCU was varied to obtain different irradiances. The exposure time was also variable to subject the specimens to the same radiant exposure (24 J/cm2). Three curing protocols were used: Standard irradiance (S): 400 mW/cm 2 for 60 s (10 mm of distance between the LCU and specimen); Medium irradiance (M): 700 mW/cm 2 for 34 s (6 mm of distance between the LCU and specimen) and High irradiance (H): 950 mW/cm 2 for 26 s (no distance between the LCU and specimen). The irradiance was checked with a radiometer (HILUX LEDMAX, SDI, Victoria, Australia).
|Table 1: Characteristics of the resin composites used in this study (information provided by the manufacturer) |
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For DC, the absorbance peak of the composite resins was obtained using the transmission mode of a Fourier Transform Infrared spectrometer (FTIR, Alpha, Bruker, Germany), operating at 24 scans and a resolution of 2 cm−1 . Five samples of each resin composite were light activated in a polytetrafluoroethylene (PTFE) mold (diameter of 5 mm and a depth of 2 mm) in accordance with the tested curing protocols (S, M and H). After photoactivation, the specimens were stored in a dark container at 37°C for 24 h. The DC was obtained on the irradiated surface. For Filtek Z250, the DC was calculated from the ratio between the peaks of the aliphatic C = C bonds (1638 cm−1 ) and aromatic C = C bonds (1608 cm−1 ) obtained from the uncured and cured specimens according to the following formula:
DC (%) = 100 × [1 − (R cured /R uncured )]
Where R = Peaks at 1638 cm−1 /peak at 1608 cm−1
The silorane based resin does not contain aliphatic C = C groups, so the spectra analyzed was between 730 cm−1 and 950 cm−1 , which corresponds with the oxirane ring-opening regions. The oxirane peak was detected at 835 cm−1 . The peak of the aromatic C = C bond (1608 cm−1 ) remained constant after light activation. The DC was calculated using the following formula:
DC (%) = 100 × (R cured /R uncured )
Where R = Peak at 835 cm−1 /peak at 1608 cm−1
To evaluate Knoop microhardness (KHN), five specimens for each experimental group were prepared in a PTFE mold with a diameter of 5 mm and a depth of 2 mm. The specimens were light activated in accordance with the experimental curing protocols (S, M and H). After 24 h of storage in a dark container at 37°C, finishing and polishing procedures were carried out using Sof-Lex discs (3 M ESPE, Saint Paul, MN). The irradiated surface of each specimen was submitted to five Knoop indentations using a microhardness tester (Micromet 2003, Buehler, Lake Bluff, IL, USA) under a load of 25 g for 15 s.
The flexural strength (FS) was obtained from the three-point bending test. A total of 10 specimens were prepared for each group using a bar-shaped steel mold (1 mm × 2 mm × 10 mm). The specimens were photoactivated according with the evaluated curing protocols (S, M and H). After storage in distilled water in a dark container for 24 h, the specimens were measured using a caliper (Digimess caliper, Mitutoyo Corp., Tokyo, Japan). The three-point bending test was performed in a universal testing machine (EMIC DL 200 MF, Pinhais, PR, Brazil) with 6 mm between the supports and at a crosshead speed of 0.5 mm/min with a load cell of 50 N. The FS was obtained from the load at fracture using the following formula:
FS = 3FI/2bh 2 (MPa)
Where F is the failure load (N); I is the support span length (6 mm); b is the specimen width (mm) and h is the specimen height (mm).
Cylindrical specimens (6 mm in diameter and 3 mm in height) were prepared for the diametral tensile strength (DTS) test (n = 10). The specimens were prepared in a steel mold and were polymerized according to the evaluated curing protocols (S, M and H) and then stored in a dark container with distilled water at 37°C for 1 week. After measuring the specimen with a caliper (Digimess caliper, Mitutoyo Corp., Tokyo, Japan), the test was performed in a universal testing machine (EMIC DL 200 MF, Pinhais, PR, Brazil), with a crosshead speed of 0.5 mm/min and a load cell of 200 N. The load at fracture was recorded in Newtons and the DTS was calculated using the following formula:
RT = 2P/DL (MPa)
Where P is the failure load (N); =3.1416; D is the specimen diameter (mm) and L is the specimen height (mm).
Data obtained in each test were evaluated by two-way analysis of variance (two-way ANOVA). Tukey's test was used for multiple comparisons with a significance level of 5%. All statistical analyzes were performed using the software "Primer Biostatistics" version 4.0 for Windows (1996).
| Results|| |
The results of the DC, KHN, FS and DTS tests are presented in [Table 2]. With respect to the DC, no statistical significant difference was demonstrated for the curing protocols, regardless of the resin composite. No statistically significant differences were found for KHN between the curing protocols for Filtek Z250. However, Filtek P90 showed a higher KHN when activated with the M irradiance when compared with the S irradiance (P < 0.05).
For FS, Filtek Z250 showed higher FS values when activated with the H irradiance when compared with the S irradiance (P < 0.05). For Filtek P90, no statistical differences were found between the curing protocols. The two-way ANOVA did not detect and statistical differences among the groups with regards to DTS.
|Table 2: Means and standard deviation of degree of conversion, KHN, FS and DTS for each experimental condition |
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| Discussion|| |
For the same radiant exposure, different combinations of irradiance and exposure time did not influence the DC of the composites, but there was an influence on some mechanical properties. Some authors also showed that different curing protocols did not influence the DC of the composites; ,,,,, although, the current data was not in agreement with other studies. ,,,, Peutzfeldt and Asmussen  speculated that a low number of experimental groups can be responsible for the non-significant results when different combinations of irradiance and exposure times were used. Indeed, extreme conditions of very low (50 or 100 mW/cm 2 ) irradiance were not tested in the present study. Clinically, this low irradiance could be present in deeper cavities; however, this present study did not intend to evaluate cavity depth.
The conversion values for P90 were low (ranging from 43.5% to 56.4%), but similar to those found in another study,  which found lower values for P90 when compared with conventional composites. Those authors speculated that the matrix and filler size, volume and type could influence the results. With respect to the mechanical properties, other studies showed a variable influence of the curing protocols, which was demonstrated in the current study. ,,
For Filtek Z250, the microhardness values were not affected by the different combinations of irradiance and exposure time, which is in accordance with the results of another study  and with the similar DC found for the curing protocols. On contrary, Filtek P90 showed higher microhardness values when activated with the medium protocol than when the standard irradiance was applied. Da Silva et al.  also found higher microhardness values when a higher irradiance was applied to conventional composites (Filtek P60 and Supreme). Those authors speculated that a higher irradiance could have developed a greater rise in exothermic heat in the composites, increasing the mobility of the monomer molecules, leading to a higher DC ,,,, and consequently, to higher microhardness values. Therefore, different values of microhardness was expected between the standard and the medium irradiance, but could be also expected between the standard and the high irradiance. However, there were no statistical differences between them.
With regards to the FS, the high protocol led to higher values for Filtek Z250 than those found for the conventional irradiance while there were no differences between the curing protocols for P90. No difference was found in another study between the curing protocols for the tested composites.  After studying different curing protocols, Lopes et al.  found higher FS values for the high irradiance protocol and speculated that the high irradiance generated a high number of centers of polymer growth,  increasing the number of cross-links in the polymer, increasing the strength of the material. The heat generated with the use of high irradiance could also accelerate the reaction kinetics,  contributing to the cross-linking of the polymer network.
The DTS was not affected by the curing protocols. It has been speculated that the test used in the current study is not sensitive enough.  Furthermore, the samples were stored for 1 week before testing in the current study. Some authors have found that a longer time of water storage could diminish the hardness of conventional composites, but not of silorane-based composites.  Therefore, the longer storage may have damaged the DTS values for Filtek Z250, but not for P90. This could be explained by the high hydrophobicity of the siloxane molecule in P90, which reduces the sorption and solubility of this resin matrix.  The results for Z250 were generally statistically higher than the results for P90, except for DTS which was not affected by the resin composite factor. However, the comparison between the two materials should be avoided since the composites tested present significant differences in composition and polymerization. Therefore, the statistical differences between the two resin composites are not included in the results. Additional studies using only the monomeric matrix of these composites could elucidate the actual influence of the monomer on the DC and on mechanical properties of polymers.
| Conclusion|| |
The influence of combinations of irradiance and exposure time for a given radiant exposure depends upon the resin composite as well as the specifically evaluated mechanical property since the different curing protocols did not influence the DC and DTS of the resin composites, but did influence the other mechanical properties. For KHN, only P90 was affected by the curing protocol. On the contrary, for FS, only Z250 was influenced by the curing protocol. Further clinical studies are necessary to add knowledge regarding the behavior of silorane-based resins.
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M R Gonzalez
Department of Restorative Dentistry, University of the State of Rio de Janeiro, Rio de Janeiro
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2]